In recent years, a demand for antenna duplexers has been increasing due to a rapid spread of mobile phones using a communication system, such as Wideband Code Division Multiple Access (W-CDMA), performing simultaneous transmission and reception. Antenna duplexers are implemented by small, low profile, and mass-productive acoustic wave elements, such as a surface acoustic wave (SAW) element, a boundary elastic wave element, or a bulk acoustic wave element.
An antenna duplexer generally includes a transmission filter and a reception filter in order to branch a transmission band signal and a reception band signal higher than the reception band signal. These filters may often employ ladder-type filters including series-arm resonators and parallel-arm resonators connected in a ladder arrangement.
In recent years, in order to eliminate an interstage filters used for a radio circuit, an antenna duplexer has required to have a higher performance than conventional duplexers. Specifically, such an antenna duplexer has been required that has improved attenuation characteristic and isolation characteristic while having an insertion loss equal to an insertion loss of the conventional duplexers. In particular, the reception filter is required to have an improved attenuation characteristic and isolation characteristic in a transmission band.
For example, Band 2 specified by the 3rd Generation Partnership Project (3GPP) standard, the transmission band ranges from 1850 MHz to 1910 MHz and the reception band ranges from 1930 MHz to 1990 MHz. This specification has a very wide passband of 60 MHz and a very narrow interval between the transmission band and the reception band. This configuration hardly provides an antenna duplexer with both of a low insertion loss and a high attenuation.
Other bands, such as Band 3 and Band 8, on the other hand, require an antenna duplexer that has a characteristic steeper and wider than that of Band 2.
Japanese Patent Laid-Open Publication No. 9-55640 discloses a ladder-type filter. In the ladder-type filter, a distance between an IDT electrode and a reflector to produce spurious emission in a one-port resonator. Japanese Patent Laid-Open Publication No. 9-55640 describes that the one-port resonator is used as a parallel-arm resonator of a filter to provide the filter with a steep characteristic in a low band-side of the band. However, the one-port resonator producing the spurious emission has high impedance between a spurious resonance frequency and a main resonance frequency. This provides the filter with a poor attenuation characteristic at a frequency causing the high impedance, thus failing to provide a high attenuation characteristic.
In a filter disclosed in Japanese Patent Laid-Open Publication No. 2001-308676, a one-port resonator producing a spurious emission has a low Q value at a spurious resonance and a high Q value at a main resonance. Japanese Patent Laid-Open Publication No. 2001-308676 describes that this reduces impedance between the main resonance and the spurious resonance, thus providing a favorable attenuation characteristic at a frequency between the main resonance frequency and the spurious resonance frequency. However, this configuration causes insufficient spurious resonance frequency at the spurious resonance frequency due to the low Q value at the spurious resonance.
Japanese Patent Laid-Open Publication No. 2000-261288 discloses a ladder-type filter in a conventional antenna duplexer. In a resonator of the ladder-type filter, electrode fingers of the IDT electrode are arranged partially at a different pitch to provide the resonator with a spurious resonance. This ladder-type filter including the resonator in a series arm has a steep characteristic at a high frequency side in the passband. Japanese Patent Laid-Open Publication No. 2000-261288 also discloses that a ladder filter including a resonator in the series arm can be applied to a duplexer. At least one electrode finger interval different from other electrode finger intervals provides another resonator having a spurious resonance.
A mere application of the techniques disclosed in the above patent documents to an antenna duplexer cannot provide an antenna duplexer having a steep and wide-band high attenuation characteristic required by Band 2, Band 3, and Band 8. Further, these patent documents do not disclose or even suggest a technique realizing the high isolation characteristic specifically required particularly for the antenna duplexer.